The present invention relates to a servomechanism driven by fluid pressure for use with various kinds of industrial machinery or industrial robots, etc.
The recent spread of industrial robots has developed a range of employment of robots, bringing about wider application thereof even in the field of assembling works which require highly elaborate techniques exercised only by skilled experts heretofore.
For the above purpose, however, it is essentially necessary to employ a manipulator that has flexible positioning and holding capability with respect to works having various configurations and that is freely maneuverable.
In the case of an electric actuator (DC servo, AC servo), since the ratio of power per weight of a motor thereof including a speed reducer is small, it is quite difficult, for example, to construct a wrist-finger having a small-size and that facilitates free movement at the end portion of a multi-joint arm of the robot.
On the other hand, a rotary actuator which uses a mechanical servo has been conventionally employed in order to control the displacement of an output shaft, the speed and the torque of an oil pressure actuator in accordance with input signals.
FIG. 5 shows the prior art rotary actuator. The actuator includes an input shaft 301, an output shaft 302, a guide valve sleeve 303, a fixed bridge 305, a rotor vane 306, a housing 307 and an oil port 308 through which working oil is supplied or discharged. The input shaft 301 is slidably accommodated within the output shaft 302. A guide valve 304 is formed between a groove formed in the outer surface of the input shaft 301 and a groove provided in the inner surface of the guide valve sleeve 303 fixed to the output shaft 302. When angular deviation is brought about between the input shaft 301 and the output shaft 302, the guide valve 304 is opened in proportion to the degree of the deviation. At this time, the motor produces torque in such direction as to compensate the deviation. As a result, the output shaft 302 is rotated, following the rotation of the input shaft 301. If the rotary actuator of the construction as described above is designed to be small in size for application, e.g., in the wrist-finger facilitating angular free movement, disadvantages as outlined below will result.
Specifically, when the rotary actuator shown in FIG. 5 is to be mounted in a joint of the finger, it is necessary that an electric motor for driving the input shaft 301 be provided adjacent to the actuator. Furthermore, if a DC motor is selected for the electric motor, encoder is required to detect angles of the input shaft 301 or the output shaft 302.
Therefore, even when the rotary actuator is desired to be small in size, such is difficult to attain because there exist limitations in the miniaturization of the DC motor and the encoder. Although the encoder can be dispensed with if a pulse motor is employed for the electric motor, in this case, it is disadvantageous in that the decomposing efficiency of the output shaft 302 is restricted by the number of poles of the pulse motor.
In the meantime, an actuator driven by oil pressure using an oil pressure servovalve has also been widely employed. However, even in the actuator of this type, the encoder is inevitably required to detect the position of the output shaft.
Accordingly, when any of the actuators driven by the electric motor and the actuator driven by oil pressure is employed, a small-sized finger of angularly free facilitation which manages approximately the same function as that of a human finger is considerably difficult to attain.